Method of forming holes

ABSTRACT

A PROCESS FOR DRILLING HOLES WITH REDUCED BURR SIZE AT THE EXIT EDGES OF THE HOLES WIHICH INCLUDES THE STEPS OF ELECTTROPHORETICALLY DEPOSITING A POLYMERIC COATING ON THE EXIT SURFACE OF THE DRILLED MEMBER PRIOR TO DRILLING, CURING THE COATING, RILLING THE HOLES AND THEREAFTER REMOVING THE CURED COATING.

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INVENTORS JOSEPH G. CUTILLO JOHN'E. LENSKY TTORNEY .eral disadvantages which add significantly to the cost of 3,5682% Patented Mar. 9, 1911 SUMMARY OF THE tNvENrtoN' The foregoing objects are attained by the present invention which generally comprises electrophoretically depositing a curable polymer on the metal foil surface of a circuit panel at the exit side of drilled holes, and thereafter curing the deposited polymer and then drilling the holes. Upon completion of the drilling, the polymeric coating is removed by dipping in a commercial stripping solution. The polymeric coating adheres well to the surface of the metal foil such that the quantity and size of burrs are significantly reduced over the use of back-up boards.

The electrophoretic deposition of the coating reduces the difficulty in handling the extra pieces of back-up board and reduces the cost of back-up material to a small fraction of that when the boards are used for the support. The process of the invention also eliminates the necessity of taping back-up boards and sharply reduces the" disposal 3,568,296- METHOD OF FORMING HOLES Joseph G. Cutillo, Endwell, and John E. Llnslty, hinghamton. N.Y., assignnrs to international Business Machines Corporation. Annuals, NIY:

Filed Jan. 14, 1969 Set. No. 79L? int. Cl. 1123p 17/00; HOSk 3/04 (1.5. CI. 29-424 8 Claims ltl ABSTRACT OF THE DISCLOSURE A process for drilling holes with reduced burr size at the exit edges of the holes which includes the steps of electrophoretically depositing a polymeric coating on the exit surface of the drilled member prior to drilling, curing the coating. drilling the holes and thereafter removing the cured-coating.

BACKGROUND OF THE INVENTION In the manufacture of printed circuit panels, interconnections extend between the circuit lines on the opposite major surfaces of a panel to electrically link selected lines. After the holes have been formed, usually by drilling. they holes through the panel which is a laminate of dielectric or insulative material with a thin conductive metal foil, Suchvas copper, on one or both f the major Surfacm costheretofore experienced with the boards The polymeric The holes are formed at preselected locations called m f t required for the Control of burr $116 hd lands which will be formed subsequently as part of a lhl'nncr l ld -"P h Permitting more conductive line' on the surface by etching the metal foil. P i t Circuit Pllncis 9 be l d SD Y y the After the holes have been formed, usually by drilling. they drlllmg f h The decrcilsc "I lhjclmcss of the b may be cleaned by passing a fluid therethrough to remove PP 'F Significantly prolongs f' Itwseparticles and smooth the edges of the hole prepara- The fol'cgPmg other l i matures and tory to being plated with conductive metal by either the mgcs of mvcmlon Y j be aplmrcmnom the *"Q electroless or clcctrolytic methods. Pins may also be more ll l detfcnpllon 0f 11 Preferred empodlmcm inserted in the holes and soldered to the circuit lands as P lnvcllllon luuslmwd m lhc p yi draw an alternative method. "18S, whercmi Although the holes may be formed through the metal 40 v h a P311131 secilollill View 9f 11 Prlntfid F P foil layers and insulative layer by etching, the most eco- Panel as It would pp P 1 i rilli g holes therein; nomical and simplest procedure is to drill the holes. One 3" I well-known common disadvantage with the drilling S- through 1 Sketches of h S e P F method, however, is the resulting burrs on the exit side t; Step5 perfofmed accordance'with the mventlon to of the hole. These burrs can loosen or be flattened during P holes m the P Show" the subsequent processing and cause short circuits be- DESCRIPTION OF A PREFERRED EMBODIMENT tween adjacent circuit lines. The usual and well-known A printed circuit panel 10 generally appears as shown method to reduce the size and number of burrs at the hole edges is t0 use a P pp sheet Which is in FIG. 1 immediately prior to the drilling operation. Laypressed adjacent the exit side of the holes during the drillcrs ll of glass fiber cloth, impregnated with an epoxy ing process. After the holes have been drilled, the back-up resin, are laminated with thin, electrically-conductive laysheet is discarded. Each board is used only once due to ers 12 of metal foil on the two major surfaces thereof. t lioning dilhculty. By P hg e drilled Panel Foil such as copper is laminated on one or both surfaces in a slurry containing abrasive particles, the holes can b depending on where circuit lines are to be formed. The placed in acceptable condition. y I foil can be selectively etched subsequent to thedrilling The use of back-up boards has generally proven salisoperation to form circuit lines. Y l factory in the control of burr size but there are still scvaccordance with the invention are illustrated in FIGS. 2 through ll. Alter lamination of the glass cloth and foil is completed. the panels are thoroughly cleaned to remove dirt and grease by dipping as indicated in FIG. 2. The bath may consist of a reverse alkaline cleaner such as K-Z, a commercially available product of the Pennsalt Chemical Corp. of Philadelphia, Pa. This is done to re move any contamination that may be present on the surfaces. An alternative is to use a conventional .degreaser as the bath. 7

The panels are removed from the cleaning bath and dipped in an acid bath (FIG. 3) for about 30 seconds, to remove any oxide that may be present on the surface of the metal foil. When copper foil is used, the bath may be dilute hydrochloric acid such as a 10% solution at room manufacture. For instance, the use of back-up boards requires the handling of extra pieces and in certain (30 situations where extremely small holes were desired, the boards mttst be individually taped to the circuit panel to prevent relative movement during drilling. Another drawback is the material cost for the back-up boards and the subsequent disposal necessitated after a board has been 5 used for drilling. In some of the drilling machines. there is a limited space between the drill tips in the raised position and the work support so that the back-up boards take up valuable space when processing multiple circuit panels for a single drilling operation. 1

It is accordingly a primary object of this invention to provide a process for drilling holes which is less expensive The processing steps of circuit panels when drilled inv temperature. The panels are then removed from the acid and rinsed in water as indicated in FIG. 4 and thereafter dried by forced air as indicated in FIG. 5. I

The panel is now ready for electrophoretic coating of a water-based polymeric resin on the conductive metal surface which will be the exit side for the drilled holes. This is shown in FIG. 6. The copper foil 12 to be coated is connected with the positive terminal of a DC. voltage source 13 as the anode, and cathode plate 14 is connected to the negative terminal of source 13. A potentiometer 15 is placed in series with the source to increase the applied voltage as coating progresses. As shown, two panels may be placed back-to-back in tank 16 so that a single surface on each is coated simultaneously with the other.

An example of a resin suitable for the drilling application is an alkyd amine known as Electrocoat IR-l540," commercially available from the Mobil Chemical Company of Cleveland, Ohio. The material as supplied contains to solids with a pH of 7.2-7.3. The consistency is adjusted with deionized water to approximately 10% solids and the pH is adjusted to fall within a range of 7.7-8.3. The alkalinity is increased by adding ammonia or organic amines such as 'N-N, dimethylethonolamine. The temperature of the coating bath is maintained at approximately 72 F. I v I Coating is accomplished by using a volt D.C. source and varying potentiometer 15 to increase the applied voltage as coating progresses. One method is to steadily increase the voltage from 0 to 40 volts over a time period of one minute. Thereafter the voltage is increased by 10 volt increments and left at each step for 30 seconds. This procedure results in a coating approximately 0.001 inch thick. In some applications, a thickness of 0.0005 inch may be sufiicient. The applied voltage may be varied in different manners and periods of time to produce the thicknesses required. Caution should be used in applying the voltage to prevent exceeding the breakdown potential of the polymer as it is being coated on the panel.

After the alkyd amine coating has been applied, the

panel is removed from the bath for curing. The panel is first rinsed in deionized water upon removal as indicated inYFIG. 7. Rinsing removes excess bath solution. Thereafter the panel is baked to cure the resin as shown in FIG. 8. For IR-1540 the coating will cure at temperatures above C., and the coating is preferablycured at a temperature of to C. for 20 minutes to crosslink the polymer. I

At this stage the control panel is ready to be drilled as shown in FIG. 9. The panel 10 is positioned on work support 20 under drills 21 which are operated in the conventional manner. The panel is placed on the support with the coated side down adjacent clearance holes 22 in the support. The clearance holes are generally 0.015 to0.0l7 inch larger in diameter than the largest diameter drills. It has been found advantageous if the work support is formed with semispherical projections 23 at each clearance hole site. These projections concentrate the supporting force near the hole site and further reduce the occurrence of burrs.

Upon completion of the drilling operation the panel is dipped 3 to 4 minutes in a stripping solution generally.

comprising methylene chloride at room temperature. (See FIG. 10.) A satisfactory solution that is commercially available is PR-491 from the Allied Finishing Specialties Co., Chicago, ill. The latter solution is effective within approximately 30 seconds at room temperature. Most photo-resist stripping solutions are usually effective but over varying times. The solution softens the coating so that light brushing can then be used to remove the coating. The stripped panel may thereafter be rinsed in deionized water, as indicated in FIG. 11. As with the usual practice, the drilled holes are now; cleaned with a slurry of silica and water to remove bits" of loose metal and glass-epoxy that may remain in the holes. Such processing is always required to clean holes when drilled with a back-up board.

The process described above reduces both the size and number of burrs at the hole edges, and also serves as a protective coating for the metal foil until drilling is completed. The coating is much thinner than the usual bacioup board so that several coated panels can be drilled simultaneously. When drilling multiple panels only the bottom panel needs a cured coating since each panel serves as a back-up for the next one above.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inventton.

What is claimed is: 1. A process for forming holes ina planar, element having an electrically conductive surface comprising the steps of:

immersing said element in an organic polymeric composition; I,

electrophoretically depositing a coating of the polymer contained in said composition on said conductive surface; and

curing said coating after the deposition thereof on said surface; drilling at least one hole through said element, said element being oriented with said coated surface at the exit end of the hole. i i 1 2. The process described -in claim .1 wherein said polymeric composition is a water-based emulsion.

3. The .process described in claim 1 wherein said polymeric composition is a .water based alkyd emulsion.

4. The process described in claim 1 wherein said deposition is sufficient to producea coating of at least 0.0005 inch when cured. I i

5. The process described in claim 1 further comprising the step of immersing the drilled element in a-stripping solution and removing said coating.

6. The process described in claim 1 wherein said deposited coating is cured by subjecting said coating to a temperature of at least 130 C.

7. The process described in claim 1 wherein said element is supported for drilling on a surface having a supporting projection thereon corresponding to each hole to be drilled, with each said projection having a clearance hole for a drill formed therein. r

8. The process described in claim 5 wherein said stripping solution includes organic solvents and said coating is removed by brushing.

References Cited v UNITED STATES PATENTS 2,961,746 11/1960 Lyman 29-s27.2x $052,019 9/1962 Strasser 29-424 

